If YPAO is setting your speakers to 75db and it sets the subs trim to -8.5db then the sub will be running +8.5dbs hot when the trim is set to 0. That's pretty hot but whatever you like.
If YPAO is not setting to 75dbs (it doesn't mine because of extreme differences between surrounds and mains) then just use your SLM to adjust everything equally, knowing that the sub, when the meter is fluctuating (as much as 2dbs) is probably closest to being calibrated to speakers on the lowest reading on meter (may depend on meter, see below), then just add whatever extra you want.
Not sure what RS SLM you have but there may be a correction file somewhere for it in the subwoofer section stickies, not sure if newest ones need any correction. most corrections are between 1-3 dbs in warble tone frequencies of sub tone, you have to add to the readings